<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
   body>div{
     width: 200px;
     border: 1px solid red;
       /*当元素所有子元素全部浮动时
       自动识别的高度为0
       后面的元素会顶上来导致显示异常
       添加 overflow: hidden;解决此异常*/
       overflow: hidden;
   }
    #d1{
      width: 50px;
      height: 50px;
      background-color: #f30707;
      float: left;
    }
    #d2{
      width: 50px;
      height: 50px;
      background-color: green;
      float: left;
    }
    #d3{
      width: 50px;
      height: 50px;
      background-color:blue;
      float: left;
    }

  </style>
</head>
<body>
<div>
  <div id="d1"></div>
  <div id="d2"></div>
  <div id="d3"></div>
</div>

</body>
</html>